The composition range of the REA1™ glasses is stated in U.S. Pat. No. 6,482,758, Nov. 19, 2002 incorporated herein by reference.
Glass materials are generally manufactured by starting with a liquid, formed by melting solid crystalline starting materials. The liquid is cooled in a way that prevents crystallization. While there are other ways to make glass, forming it from the liquid provides a simple way to achieve large pieces of material that can readily be formed into products. Here we show that by virtue of their optical, mechanical and thermal properties and the ability to fabricate the glasses by casting from a liquid, the REA1™ glasses provide a novel material for the gain medium used to construct infrared laser devices and for optical elements such as windows and lenses.
It should be noted that certain fabrication, coating, and other operations that are well-known in the art are typically employed to prepare components of devices from the glass optical materials and optical gain media of this invention.
Lasers that produce infrared light (“infrared lasers”) are widely used in materials processing, optical communications, medical and dental diagnostics and surgical procedures, optical range finding and remote sensing, and numerous applications in analysis, marking, scribing, engraving and optical diagnostics. High power density lasers that provide a quality beam profile at infrared wavelengths are useful in materials processing operations including welding, metal cutting and metal forming operations, and medical procedures. Infrared lasers are also used in military applications for range finding, target designation, and missile guidance systems. Infrared lasers also have application in Homeland security, where sensors, laser-based detection, and laser-based defense systems that employ infrared lasers and laser technologies are being developed.
Many solid state lasers, for example the “neodymium:YAG” laser, employ trivalent rare earth ions distributed in a medium such as a crystal or a glass material that can be “pumped” to excite the laser active ions. Neodymium, erbium and ytterbium are widely used to generate light at infrared wavelengths. The gain medium provides a host for the laser active ions and forms a critical component of the laser. The gain medium must be able to transmit light at the laser wavelength with minimal losses. It may also provide a means to extract heat generated by the optical processes, and in some instances it provides a structural element of the laser itself. The gain medium may also be formed as the laser cavity by placing reflective coatings on various surfaces. Solid state lasers that employ a REA1™ glass doped with optically active species are within the scope of this invention.
The advent of high power density lasers based on Yb-doped Yttrium Aluminum Garnet (YAG) crystals containing several percent ytterbium has shown the utility of Yb lasers that can be pumped over a narrow wavelength range by using commercially available infrared laser diodes. Ytterbium ions are a desirable dopant for laser applications because, unlike other optically active rare earth ions, electronically excited Yb ions do not suffer from energy-sapping cross relaxation and excited-state absorption processes. Pumping the strongly absorbing 2F7/2 state in trivalent Yb ions with laser diodes overcomes the limitation of low pump absorption with the broadband lamp pumping schemes commonly used in Nd-based lasers. The close spacing of the absorption and emission bands in Yb3+ results in small conversion losses.
While the Yb lasers were first demonstrated as flashlamp-pumped devices in 1965, it is only recently that these lasers have acquired technological significance, through advances in pump sources, laser gain media, and laser output power that can be achieved. Small, diode-pumped Yb-doped rod lasers were first demonstrated at the Lincoln Laboratory around 1990. Subsequent laser development at Lawrence Livermore National Laboratory, Raytheon and other laboratories in the US and abroad has increased the power output of small (˜5 mm diameter, 10 mm length) rod lasers towards 1 kW to provide an enormous specific power. The thin disk Yb:YAG laser was pioneered in Germany. Power output of ˜650 Watts has been demonstrated in 0.2 mm thickness disks pumped in a region a few millimeters in diameter. The disk laser is predicted to enable a power output of ca. 10 kW from a single small disk laser device. By providing a larger planar surface for heat extraction than is possible in a long cylinder, the disk laser has potential to achieve the maximum possible power density. The wide availability of inexpensive and electrically efficient InGaAs-based laser diodes which operate in the 940-980 nm pump wavelength range needed to realize Yb-based lasers has laid the foundation for new near IR power laser products. Optical efficiencies of around 50% are achieved in disk laser configurations operating near room temperature; even higher efficiencies have been obtained using cryogenically cooled disks.
The present invention provides novel glass host materials for the Yb ions, i.e., the “REA1™” glasses comprised of rare earth oxides and aluminum oxide, that are used to make Yb: REA1™ glass laser devices. Technical drawbacks of crystalline Yb:YAG lasers relative to the lasers of the present invention are: (i) the Yb3+ absorption band typically necessitates pumping at around 940 nm, rather than 980 nm where inexpensive and powerful diode laser pump sources are available, (ii) pumping at 940 nm rather than 980 nm, in combination with laser emission at a wavelength of ˜1030 nm, leads to increased heat generation which limits the total power density that can be achieved, (iii) the smaller magnitude of the ground state absorption in Yb:YAG, reduces the efficiency of pump power utilization, and (iv) strain-induced birefringence in melt grown crystals due to growth stresses and lattice strain can produce beam deflection and instability in the laser cavity.
Lasers and devices that transmit infrared radiation that are based on REA1™ glasses also have potential cost advantages over the YAG- and other crystalline host-based devices because the glass forming operations are relatively inexpensive compared with crystal growing operations.
The use of REA1™ glasses for windows, lenses, filters, and other optical applications that require infrared transmitting material benefits from (i) the large Abbe number, (ii) the range of Abbe numbers, and (iii) the IR transmission to wavelengths of ˜5000 nm, and (iv) the large refractive index of these materials. The REA1™ glasses provide superior values of these properties relative to the familiar silicate glasses. The REA1™ glasses also provide thermal, chemical, and environmental stability that is superior to other infrared transmitting materials such as fluoride and tellurite glasses.